1. Field of the Invention
The present invention relates to power amplifiers and more particularly to power amplifiers in battery powered handsets, and even more particularly to such power amplifiers exhibiting two or more power paths.
2. Background Information
Radio-frequency (RF) signals generated at a mobile handset generally are amplified, transmitted through the handset antenna and sent to a base station for distribution to receivers. Often the frequency bands of operation of the handsets are predetermined, mainly in the frequency range from 800 MHz to 2000 MHz for various mobile standards such as WCDMA (wide band code division multiple access) and CDMA (code division multiple access). The present invention, however, may find advantageous use in device operating at other frequencies and with other formats.
In general, the handset is required to transmit at a high output power level when it is farther away from a receiving base station in order to maintain a pre-determined signal strength at the base station for sufficient reception. Conversely, the closer the handset to the base station, less transmitted power would be required. The handset output power is adjusted according to the command embedded within the RF control signal transmitted from the base station to the handset.
The handset transmitted signal, and hence the RF power amplifier output signal, has to meet the FCC regulation on spectral re-growth (also known as linearity—often measured in terms of adjacent channel leakage power ratio (ACLR) which stipulates the maximum allowable interference to other frequency channels in order to minimize interference between signals). Some known mobile devices (handsets) have RF power amplifiers powered by the full battery voltage at all times. The RF power amplifies are generally designed to meet the linearity specification at maximum transmit power level (+28 dBm for WCDMA system) under such a bias condition. Statistically, power amplifiers transmits at maximum linear output power only for a small fraction of time, while most of the transmissions take place at a considerably lower power levels (10-20 dB below maximum power).
The actual output power level from the power amplifier (and hence the handset), is continuous from some −50 dBm to 28 dBm. Multi-mode power amplifiers, compared to conventional single-path amplifiers, consume less current at low power outputs. Multi-mode handset power amplifiers are commonly implemented with two power modes, High Power (HP) and Low Power (LP). The HP mode generally applies to the range from 16 dBm to 28 dBm, and the LP mode applies to power levels below 16 dBm. The present invention is directed at multi-mode power amplifiers which are implemented with two (or more) power paths whereas one path delivers power for HP mode while the other path delivers power for LP mode.
FIG. 1 illustrates a dual path power amplifier 10 with a higher power path 12 and a lower power path 14. As mentioned above, in other designs additional parallel power paths may be found. The higher power path may be used where the design requires an output power from about 16 to 28 dBm, and the lower power path may be used for power less than 16 dBm. One direct effect of the two (or more) power path design is that the characteristics associated with each path are different since the electronic components in each path are different. For example, the active transistor sizes and DC currents in each path are different due to different power handling requirement for each path. These result in different electrical and thermal responses between the two paths. More specifically, the two paths experience different gain variations over temperature, resulting in a gain mismatch between the two paths. FIG. 2 illustrates the difference in gain (gain delta) between the two paths over temperature at an output power level of 16 dBm (the cross over power between the two paths), where the effect of the change in gain delta between the higher and the lower power paths over temperature will be most apparent. The two traces are shown over a frequency range from 1920 to 1980 GHz. The gain delta between the two paths is about 2.9 dB at 25° C. and 3.9 dB at 85° C. This represents an increase in gain delta by 1 dB between the two paths when the temperature increases from 25° C. to 85° C.
It is known in the art of power amplifier design that the power gain of power transistors used in an RF amplifiers decreases with increasing operating temperature and/or the junction temperature of the transistors. Therefore, apart from a change in ambient temperature, similar gain delta response shown in FIG. 2 can also be induced by a change in the junction temperature of the power amplifier. The increase in junction temperature is usually associated with high output power operation. For example, at an output power of 28 dBm, the junction temperature of the power transistors will be increased due to higher dissipated DC power, while the junction temperature of the same power transistors will be lower at a lower output power (e.g. 16 dBm).
All 3G handsets (a designation known in the art) are subjected to an inner loop power control test which is part of a standard qualification process. The test requires the handset to adjust its output power in accordance with the control commands. A portion of the test requires the output power to ramp down in 1 dB step from maximum handset transmitting power to the minimum power level and in reverse direction as shown in FIG. 3A. The handsets are generally pre-calibrated with a software look up table whereas specified input power is mapped to a specified output power for either the ramp up or ramp down mode. A baseband controller is used to adjust the power inputs to the power amplifier. Since the power amplifier starts out at a high output power level, the junction temperatures of the higher power transistors starts out at a high temperature and then gradually decreases as the output power drops during the ramp down operation. During the ramp up portion, exacerbated by the cross over from the lower power path to the higher power path at 16 dBm, the higher power output transistors start out with lower junction temperatures. The net result is a non-symmetrical response illustrated in FIG. 3B where item 30 shows a step higher than 1 dB. That higher step is directly related to the gain delta 20 mismatch over temperature shown in FIG. 2 that is due to the different junction temperatures in the transistor amplifiers present in the two paths. The gain change is approximately equal to the gain delta of FIG. 2, depending on the actual junction temperature at the time of the switching between the lower and the higher power paths. The present invention provides an improved power amplifier characteristic with respect to the gain delta of FIG. 2.
Some prior art conventional RF power amplifier designs have only a single power path using the same transistors over the entire output power range. In such an instance, there will be no temperature induced gain delta due to mismatch in junction temperatures, as found in an amplifier with two paths. The single power path operation avoids the switching from a power transistor at higher junction temperature to another power transistor at lower junction temperature.
The present invention is directed at reducing the mismatch in gain variation over temperature for the intrinsic amplifiers in each of the multiple power paths.